The present invention relates generally to electronic filters and more particularly to an active electronic filter which, using the same components, may serve as either a band-reject or a band-pass filter having the same natural frequency.
Filters, as such, are well known in the art. Two well-known types are the band-reject filter and the band-pass filter. The band-reject filter acts to pass all frequencies of its input signal except those of a particular bandwidth which frequencies are attenuated to a degree depending upon the desired use of the filter. A band-pass filter operates in the opposite manner in that it passes a band of frequencies and rejects the rest. In accordance with customary terminology as applied to filters, the center frequency of the band which is either passed or rejected is known as the natural frequency of the filter. Normally, the design of the filter is an individual process with the design being tailored to the individual need. Thus, the production of a variety of filters entails the availability of a large variety of components.
Historically, filter circuits were comprised of resistance-inductance-capacitance networks which had problems of insertion loss, critical tuning, etc. More recently, active filters comprised of networks using operational amplifiers have enjoyed great popularity. In this regard, reference is made to the book Operational Amplifiers, copyrighted in 1971 by Burr-Brown Research Corporation (Library of Congress Catalog No. 74-163297) and particularly to Chapter 8 of that book which discusses active filters.
Notwithstanding the advent of operational amplifier active filters, the individual design requirements and the problems attendant thereto have remained. In addition, certain problems such as those of sensitivity are not eliminated and in this regard attention is drawn to the staff article "Sensitivity--Key to Analog Active Filters" found in the November 1970 issue of "Electronic Design News" (pages 17 through 25).